1. Field
Embodiments of the present invention relate to laser systems and, in particular, to tunable external cavity diode lasers systems.
2. Discussion of Related Art
An optical telecommunication system transmits information from one place to another by way of an optical carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, light beam, or a lightwave signal.
The optical telecommunication system includes several optical fibers and each optical fiber includes multiple channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. The purpose for using multiple channels in the same optical fiber (called dense wavelength division multiplexing (DWDM)) is to take advantage of the unprecedented capacity (i.e., bandwidth) offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap. The International Telecommunications Union (ITU) currently determines the channel separations.
One link of an optical telecommunication system typically has a transmitter, the optical fiber, and a receiver. The transmitter has a laser, which converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
External cavity lasers (ECL) are common light sources. While such lasers have typically operated at a single wavelength or channel, tunable lasers have been recently developed that can address many channels, for example, at least all channels in one of the communication frequency bands specified by the ITU.
A typical tunable external cavity laser includes a laser diode that emits a light beam. A lens collimates the light beam and focuses it onto an (one or more) etalon. The etalon is a wavelength selective element that selects a particular wavelength from the light beam and passes only that wavelength. The tunable external cavity laser includes an end mirror. The light beam with the selected wavelength is incident on the end mirror and reflects back into the cavity, providing optical feedback that is essential for the functioning of an external cavity laser.
The laser diode has a front facet coated with an anti-reflective (AR) material that minimizes optical loss of the light coupled to the lens. The laser diode has a back facet coated with a highly reflective material. The end mirror of the external cavity and the reflective back facet of the laser diode form a cavity in which light at the selected wavelength is reflected back and forth. The light is amplified in the process and a light beam at the selected wavelength is output by the tunable external cavity laser. This is sometimes called the operating wavelength.
One of the simplest ways to change the operating wavelength is by thermal tuning. One known thermally tuned etalon has a platinum heater strip coupled to the surface of the active region of the etalon. The platinum strip heats up when current is applied and the heat is dissipated throughout the etalon. When the etalon is heated, its wavelength selectivity characteristics change and it passes a different wavelength. When the wavelength that the etalon passes changes the laser operating wavelength changes.
One problem with this known thermally tuned etalon design is that the platinum heater strip tends to degrade at high temperature and/or high current densities. The degradation accelerates each time there is a change in temperature of the platinum strip to accommodate changing of the operating wavelength of the laser. Degradation of the platinum heater strip results in poor control of the operating wavelength. If the platinum heater strip ultimately degrades to the point at which it eventually fails, the ability to control the operating wavelength of the laser is lost. Platinum heater strip degradation can therefore present a reliability issue and thus become a limiting factor in designing and operating this type of known thermally tuned laser.